1. Field of the Invention
The invention relates in general to carbon fiber-carbon matrix reinforced ceramic composites and the production of such composites.
2. Description of the Prior Art.
Carbon fiber reinforced carbon matrix composites have been widely used for high temperature applications, particularly in military and aerospace applications. Such carbon reinforced carbon matrix composites have not achieved universal acceptance for such applications because they suffer from certain inherent limitations. Reinforced ceramic matrix composites, such as, for example, silicon carbide reinforced ceramic matrix composites had previously been proposed for high temperature applications, but such composites also suffer from certain limitations, which restrict their general use.
Carbon fibers retain their strength at elevated temperatures, but in the presence of oxygen they oxidize rapidly at temperatures above about 900 degrees Fahrenheit. To be useful at elevated temperatures carbon fibers must be protected from oxidation. Materials that are capable of resisting destructive oxidation at ultra-high temperatures (above approximately 1500 degrees centigrade) are typically ceramics. Such ceramics are potentially useful as matrix materials in composites and they include, for example, zirconium and hafnium carbide, but they suffer from other problems that had previously appeared to disqualify them for general use. For example, although zirconium carbide oxidizes in the presence of oxygen at elevated temperatures, above about 1900 degrees centigrade the resulting zirconium oxide coating is dense and tightly adhered to the remaining body of zirconium carbide. Once a dense coating of zirconium oxide forms it protects the underling zirconium carbide matrix from further oxidation so long as the temperature remains above about 1900 degrees centigrade. Unfortunately, below about 1900 degrees centigrade zirconium oxide becomes a loosely adhered powder that falls off the substrate, thereby leaving the underling zirconium carbide exposed which in turn oxidizes and falls off. Oxidation rapidly consumes the entire body of zirconium carbide below about 1900 degrees centigrade. Between approximately 1900 and 2500 to 2700 degrees centigrade zirconium carbide provides a very satisfactory matrix for composites because of the tightly adhered oxide coating. Hafnium carbide likewise provides a satisfactory matrix for carbon fibers at high temperatures from about 1900 degrees centigrade up to at least approximately 2700 degrees centigrade, but fails rapidly below about 1900 degrees centigrade. Hafnium carbide is, however, much higher in density than zirconium carbide. This weight penalty (approximately 50 percent greater) makes hafnium carbide undesirable for many applications. Silicon carbide is a satisfactory matrix for carbon fibers only up to a temperature of approximately 1650 degrees centigrade. It fails rapidly above about this temperature. When an application for a composite requires prolonged operation in an oxidizing environment at temperatures between about 1650 and 1900 degrees centigrade, the results are generally not fully satisfactory because the available matrix materials tend to fail rapidly in this temperature region.
Molten silicon tends to diffuse very aggressively into other materials. When silicon encounters carbon fibers or carbon matrix materials in a carbon-carbon matrix composite, the silicon tends to rapidly diffuse into the carbon and convert it to silicon carbide. This limits the usefulness of silicon in composites where carbon fibers are present.
The melt infiltration method used here is a pressureless process in which a molten material wicks by way of capillary action into a fiber preform. As compared with high pressure composite forming processes, the costs of equipment and operation are much lower for this pressureless melt infiltration process. If reactants are present, the infiltrating molten material will react in situ with the reactants during the infiltration process (infiltration reaction). It had previously been proposed to infiltrate a tightly woven carbon fiber preform containing sacrificial carbon with molten silicon (melting point of about 1414 degrees centigrade) so as to form a dense matrix of silicon carbide. It was proposed to protect the carbon fibers from reacting with the molten elemental silicon by the presence of a dense oxide coating on the carbon fibers. See, for example, Brun et al. U.S. Pat. No. 5,552,352, which proposes coating individual carbon or silicon carbide fibers with two coats, first, a metal oxide coat, and, second, a carbide, nitride, silicide, diboride, or noble metal coating over the metal oxide. All of the fibers have both coatings, that is, the coatings are both applied at the fiber level. A silicon carbide matrix is formed by the melt infiltration of molten silicon. The function of the second layer is said to be the prevention of a reaction between the fibers and the molten silicon. The molten silicon infiltrates a porous preform and forms a matrix in contact with the outer coating on each of the fibers according to Brun et al. It does not diffuse into a pre-existing solid carbide matrix. The resulting composite according to Brun et al. is a fiber reinforced ceramic matrix. It is not a carbon-carbon composite as the reinforcement within a ceramic matrix. Kameda et al. U.S. Pat. No. 6,235,379 proposes a silicon carbide fiber reinforced silicon carbide matrix in which the silicon carbide fiber is coated and the amount of free silicon next to the bundles of fibers is minimized as compared with the rest of the silicon carbide matrix. The silicon carbide matrix would fail rapidly at temperatures above about 1650 degrees centigrade. Heine et al. U.S. Pat. No. 6,231,791 proposes that graphite fibers be coated with two layers of graphite, and embedded in a body of carbon which is then melt infiltrated with silicon and reacted to silicon carbide. It is proposed to limit the exposure of the fibers to silicon at the infiltration temperature so as to prevent them from being attacked by the molten silicon.
A low temperature chemical vapor deposition method for the formation of dense protective coatings of materials such as titanium dioxide, zirconium dioxide, hafnium dioxide, tantalum oxide, alumina, and the like on carbon fibers within a bundle of carbon is disclosed in Zinn et al. Ser. No. 09/979,929 filed Nov. 27, 2001, now U.S. Pat. No. 6,921,707, and assigned to the same assignee as this application. Such protective oxide coatings are well suited to protecting the carbon fibers from reaction with molten materials in melt infiltration operations.
Various proposals had been made for producing fiber reinforced ceramic composites. See, for example, Kawai et al U.S. Pat. No. 5,254,397, which proposes that the exterior of a substrate composed of a ceramic or carbon matrix reinforced with carbon fibers should be coated with a graded coating layer. The proposed graded layer would grade from silicon carbide on the exterior to the composition of the ceramic matrix on the inside. This graded layer is on the composite, not the individual fibers. The proposed silicon carbide faced composite would likely fail above about 1650 degrees centigrade. The silicon is not melt diffused into the composite so there is little risk that it will attack the carbon fibers during formation of the composite. Kaplan et al. U.S. Pat. No. 5,283,109 discloses the deposition of alternate thin layers of silicon carbide and zirconium or hafnium carbide on a carbon-carbon substrate to protect it up to about 3500 degrees Fahrenheit (about 1925 degrees centigrade).
The performance of many operations such as, for example, high temperature combustion chambers could be significantly improved if they could operate at temperatures of between room temperature and approximately 2500 to 2700 degrees centigrade, and particularly if this could be accomplished with lightweight materials. Being able to operate at any temperature over the full temperature range from room up to ultra-high temperatures with relatively inexpensive structures would greatly expand the flexibility of use and the potential markets for many structures, including, for example, the hot sections of commercial jet engines, reactors, furnaces, and the like. Those skilled in the art have recognized the need for a carbon fiber reinforced composite that will retain its useful characteristics over a wide range of operating temperatures, and particularly from approximately 1000 to 2700 degrees centigrade.
These and other difficulties of the prior art have been overcome according to the present invention.